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From "T Jake Luciani (JIRA)" <>
Subject [jira] Commented: (CASSANDRA-1470) use direct io for compaction
Date Fri, 10 Dec 2010 16:39:02 GMT


T Jake Luciani commented on CASSANDRA-1470:

.bq see for why posix_fadvise
won't work [for writes]

This article is talking about NOREUSE flag being a no-op but we are using DONTNEED which does

Since the true goal of this ticket is to minimize the performance impact of compaction I'd
like to try the following:

At BRAF level:
   * use fadvise(DONTNEED) instead of direct-io for writes. This will fix the buffering problem
we now see affecting write speed.  
   * use fadvise(DONTNEED) for sstable reads to remove the need for directio flag altogether.
   * add a method long[] pagesInPageCache()  which uses the posix mincore() function to detect
the offsets of pages for this file currently in page cache.

At Compaction level(a separate ticket):
   * add getActiveKeys() which uses underlying pagesInPageCache() to get the keys actually
in the page cache
   * use getActiveKeys() to detect which SSTables being compacted are in the os cache and
make sure the subsequent pages in the new compacted SSTable are kept in the page cache for
these keys. This will minimize the impact of compacting a "hot" SSTable as well. 

A simpler yet similar approach is described here:

> use direct io for compaction
> ----------------------------
>                 Key: CASSANDRA-1470
>                 URL:
>             Project: Cassandra
>          Issue Type: Improvement
>          Components: Core
>            Reporter: Jonathan Ellis
>            Assignee: Pavel Yaskevich
>             Fix For: 0.7.1
>         Attachments: 1470-v2.txt, 1470.txt, CASSANDRA-1470-for-0.6.patch, CASSANDRA-1470-v10-for-0.7.patch,
CASSANDRA-1470-v11-for-0.7.patch, CASSANDRA-1470-v12-0.7.patch, CASSANDRA-1470-v2.patch, CASSANDRA-1470-v3-0.7-with-LastErrorException-support.patch,
CASSANDRA-1470-v4-for-0.7.patch, CASSANDRA-1470-v5-for-0.7.patch, CASSANDRA-1470-v6-for-0.7.patch,
CASSANDRA-1470-v7-for-0.7.patch, CASSANDRA-1470-v8-for-0.7.patch, CASSANDRA-1470-v9-for-0.7.patch,
CASSANDRA-1470.patch, use.DirectIORandomAccessFile.for.commitlog.against.1022235.patch
> When compaction scans through a group of sstables, it forces the data in the os buffer
cache being used for hot reads, which can have a dramatic negative effect on performance.

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